struct ListNode* deleteDuplicates(struct ListNode* head)
 {
    if (!head)  
    {
        return head;
    }
    struct ListNode* cur = head;
    while (cur->next) 
    {
        if (cur->val == cur->next->val) 
        {
            cur->next = cur->next->next;
        } 
        else
        {
            cur = cur->next;
        }
    }
    return head;
}